Abstract

Reprogramming strategies allow for the generation of virtually any cell type of the
human body, which could be useful for cell-based therapy. Among the different reprogramming
technologies available, direct lineage conversion offers the possibility to change
the phenotype of a cell type to another one without pushing cells backwards to a plastic/proliferative
stage. This approach has raised the possibility to apply a similar process in vivo in order to compensate for functional cell loss. Historically, the cerebral tissue
is a prime choice for developing cell-based treatments. As local pericyte accumulation
is observed after central nervous system injury, it can be reasoned that this cell
type might be a good candidate for the conversion into new neurons in vivo. In this article, and by focusing on recent observations from Karow and colleagues
demonstrating the possibility to convert human brain-derived pericytes into functional
neurons, we present a brief overview of the state of the art and attempt to offer
perspective as to how these interesting laboratory findings could be translated in
the clinic.

Commentary

Reprogramming strategies are highly promising for the development of future regenerative
medicine approaches. Over the past 60 years, from the first nuclear transfer experiment
to the description of cell fate conversions by forced expression/inhibition of cell-type
specific transcription factors (TFs) and/or miRNAs, a great deal of evidence showing
cell identity switches has been accumulated [1]. Numerous cocktails of TFs have to date been identified for the conversion of somatic
cells all the way back to pluripotency or into more specialized cell types. Accordingly,
the emergence of cell fate conversion strategies represents a new unexpected source
for generating any desired cell type that could be of interest for the clinic. These
lineage conversion strategies could be divided into: direct conversion approaches,
in which tissue-specific TFs are employed and characterized by the lack of proliferation
and/or progenitor generation; and indirect conversion approaches, based on the use
of so-called pluripotency factors. Indirect lineage conversion, as opposed to direct
conversion, is driven by a partial dedifferentiation step, with potential for the
generation of expandable progenitor populations, followed by redifferentiation into
specific populations (for a recent review see [1]).

Noticeably, cerebral tissue has always been one of the most studied tissues for the
development of cell-based therapies, sustained by the hope that cell transplantations
could facilitate the replacement of lost neural tissue and/or protect from neurodegenerative
processes. Although several cell types have demonstrated encouraging results in animal
models, neural stem cells and their derivatives are naturally seen as the most suitable
cellular products. Taking into account that access to such cellular material is technically
and ethically difficult, the advent of lineage conversion strategies has opened new
vistas towards the generation of neural cells as well as towards the exploration of
new treatment avenues.

In 2010 the Wernig laboratory was the first to meet this challenge by defining a set
of three TFs - that is, Ascl1, Brn2 and Myt1l - allowing for the generation of functional
neurons upon overexpression in mouse fibroblasts [2]. Later on, the same group demonstrated that addition of NeuroD1 to this cocktail
was sufficient to achieve the same conversion in human fibroblasts [3]. Numerous reports have since then reproduced and extended this discovery by identifying
other lineage specifiers able to convert fibroblasts into neural progenitors or more
specialized neurons such as motoneurons [4] and dopaminergic neurons [5]. Alternatively, by combining pluripotency-associated reprogramming factors with neural
specifiers or defined media, other laboratories have reported that it is feasible
to convert fibroblasts into expandable neural stem cells [6].

From these and other reports, two main lineage conversion strategies can be envisaged
for clinical applications: the generation of the desired cell type(s) in vitro and its further transplantation; and the local in vivo conversion of one cell type into the one(s) required, in a similar way to what has
been experimentally done with pancreatic cells. As a common denominator for most of
these studies, however, fibroblasts represent the starting cell type of choice for
achieving this lineage conversion; cord blood cells [7] and astrocytes [8] are exceptions to this rule. The latest strategy might thus not be the most appropriate
unless other cell types present in the brain, and not as essential for brain functions
as neural cells, could be reprogrammed.

Interestingly, a recent report from the Berninger laboratory convincingly extended
the spectrum of somatic cell types able to give rise to neurons upon forced expression
of TFs [9]. In their report, Karow and colleagues describe the in vitro conversion of pericytes, residing in the adult human brain, using only two TFs: Sox2
and Mash1 (Ascl1). The authors showed that the so-called human pericyte-derived induced
neurons acquired molecular marks and electrophysiological properties resembling those
of primary mature neurons, with further maturation when co-cultured with murine neurons.
Even though the final evidence for the phenotypic identity of the newly generated
neurons is not yet provided, they seem to display the hallmarks of GABAergic interneurons.
Of note, the authors observed a conversion rate of about 20% but also an important
cell death (≈30%) in the cotransfected cells. Although the mechanism of action controlling
this lineage conversion is not fully identified, Sox2 being a pluripotent factor as
well as a neural lineage specifier, a synergic effect may occur consisting of Sox2induced
partial dedifferentiation alongside activating a neuronal program with both Sox2 and
Mash1. Of importance, the authors showed that converted cells do not undergo cell
division during this process, supporting the idea that this occurs via a direct conversion
mechanism.

In conclusion, pericytes not only represent a new cell type suitable for neuronal
conversion but also provide a new cell candidate for in vivo approaches. Indeed, similarly to astrocytes, pericytes have been identified as one
of the core components of post-injury scarification in the central nervous system
[10]. Considering that pericytes are amenable to being converted into neurons, we can
imagine that these cells, localized within or close to an injured area, may be a good
target for in vivo conversion without much effect on the surrounding neural cells. However, it will be
important to specifically convert pericytes in vivo but also to limit the cell death associated with this process. The role of resident
pericytes after injury should also be precisely studied to avoid any side effect upon
conversion. Importantly, both central nervous system injuries and neurodegenerative
processes generally affect more than one cell type, implying that several specialized
cells counteracting the effects observed in the diseased brain might be needed.

All in all, the study by Karow and colleagues presents pericytes as a new contender
in the race to the operating room but we have a long way to go before a particular
cell type gets elected.

Abbreviations

miRNA: microRNA; TF: transcription factor.

Competing interests

The authors declare that they have no competing interests.

Acknowledgements

The authors thank May Schwarz for administrative support. The laboratory of JCIB is
supported by grants from Fundacion Cellex, CIBER the G Harold and Leila Y Mathers
Charitable Foundation, The Leona M and Harry B Helmsley Charitable Trust, and MINECO.